Recirculating ball worm drives have been proposed in which a groove is cut into the worm nut so that the balls can be partly retained in the groove in the nut. One or more return ducts are formed in the nut, depending on the load which can be transmitted by the drive. The return duct is so arranged that the end portion of one helical path cut in the nut extends to another terminal portion of the nut, for example close to the starting portion of the same helical turn. The return ducts have usually been made as separate elements, fitted in suitable recesses or bores formed in the nut. It is difficult to manufacture such duct portions. The surface of the nut which defines the circulating path for the balls must be accurately machined in order to prevent play, and to provide accurate guidance to the balls, free from undesired clearances. Consequently, manufacturing such a spindle nut was expensive, since internal grinding was necessary. Assembly of the nut about the spindle, with the balls properly located, is complex and hence expensive. Careless handling of the spindle drive may result in excess movement of the nut to an end portion of the spindle, so that balls might fall out of the grooves formed in the nut and in the spindle, causing loss of some of the balls. Only a very limited selection of material is available for the nut, since the helical groove for the balls in the nut must be hard and strong, to permit transfer of forces between the nut and the spindle without deformation of the grooves in the nut and in the spindle, in which the balls run.